Search

Supercooled liquid sulfur provides higher capacity, faster kinetics, and better cycling life compared to solid sulfur., In lithium-sulfur (Li-S) chemistry, the electrically/ionically insulating nature of sulfur and Li2S leads to sluggish electron/ion transfer kinetics for sulfur species conversion. Sulfur and Li2S are recognized as solid at room temperature, and solid-liquid phase transitions are the limiting steps in Li-S batteries. Here, we visualize the distinct sulfur growth behaviors on Al, carbon, Ni current collectors and demonstrate that (i) liquid sulfur generated on Ni provides higher reversible capacity, faster kinetics, and better cycling life compared to solid sulfur; and (ii) Ni facilitates the phase transition (e.g., Li2S decomposition). Accordingly, light-weight, 3D Ni-based current collector is designed to control the deposition and catalytic conversion of sulfur species toward high-performance Li-S batteries. This work provides insights on the critical role of the current collector in determining the physical state of sulfur and elucidates the correlation between sulfur state and battery performance, which will advance electrode designs in high-energy Li-S batteries.

To make a lithium-sulfur (Li-S) battery practical, not only high gravimetric energy capacity is important, but also high volumetric energy capacity will be required. The currently explored Li-S cathode designs often deploy systems with liquid electrolyte infiltration, hence with relatively low volumetric capacity. In the current study, we theoretically test a compact solid three-dimensional (3D) design (more like a Li-ion battery cathode than a conventional Li-S cathode) consisted of a sandwich structure alternating between the two-dimensional (2D) Mn-hexaaminobenzene-based coordination polymer (2D Mn-HAB-CP) layer and the amorphous Li-S layer. We study the theoretical limits for both its gravimetric and volumetric energy capacity, as well as its structural stability and Li diffusion within the cathode system. To study the Li diffusion within an amorphous system, we also develop a pull-atom molecular dynamics (PA-MD) to calculate the barrier heights of such disordered systems. We reveal the mechanism that determines the Li diffusion in the amorphous layer of the system. Overall, we find such a 3D solid Li-S cathode can be practical, with sufficient large gravimetric and volumetric energy capacity, as well as the Li diffusion constant. It also solves many other common Li-S cathode problems, from Li polysulfide dissolution to electrical insulating, and structure instabilities.

A crystalline solid electrolyte interphase Li

Charged-defect calculation using a periodic supercell is a significant class of problems in solid state physics. However, the finite supercell size induces an undesirable long-range image charge Coulomb interaction. Although a variety of methods have been proposed to eliminate such image Coulomb interaction, most of the previous schemes are based on a rough approximation of the defect charge screening. In this work, we present a rigorous derivation of the image charge interaction with a new defect screening model where the use of bulk macroscopic dielectric constant can be avoided. We have verified this approach in comparison with a widely used approach for 12 different defects. Our correction scheme offers a much faster convergence concerning the supercell size for cases with considerable image charge interactions. In those cases, we also found that the nonlinear dielectric screening might play an important role. The proposed new defect screening model will also shed new light on understanding the defect screening properties and can be applied to other defect systems.

Heat-induced perovskite LED device degradation is elucidated via insightful in situ structure and spectroscopy characterizations., Achieving perovskite-based high–color purity blue-emitting light-emitting diodes (LEDs) is still challenging. Here, we report successful synthesis of a series of blue-emissive two-dimensional Ruddlesden-Popper phase single crystals and their high–color purity blue-emitting LED demonstrations. Although this approach successfully achieves a series of bandgap emissions based on the different layer thicknesses, it still suffers from a conventional temperature-induced device degradation mechanism during high-voltage operations. To understand the underlying mechanism, we further elucidate temperature-induced device degradation by investigating the crystal structural and spectral evolution dynamics via in situ temperature-dependent single-crystal x-ray diffraction, photoluminescence (PL) characterization, and density functional theory calculation. The PL peak becomes asymmetrically broadened with a marked intensity decay, as temperature increases owing to [PbBr6]4− octahedra tilting and the organic chain disordering, which results in bandgap decrease. This study indicates that careful heat management under LED operation is a key factor to maintain the sharp and intense emission.

Recent experiments have shown that CO reduction on oxide derived Cu nanoparticles (NP) are highly selective toward C2+ products. However, understanding of the active sites on such NPs is limited, because the NPs have ∼200 000 atoms with more than 10 000 surface sites, far too many for direct quantum mechanical calculations and experimental identifications. We show here how to overcome the computational limitation by combining multiple levels of theoretical computations with machine learning. This approach allows us to map the machine learned CO adsorption energies on the surface of the copper nanoparticle to construct the active site visualization (ASV). Furthermore, we identify the structural criteria for optimizing selective reduction by predicting the reaction energies of the potential determining step, ΔEOCCOH, for the C2+ product. Based on this structural criterion, we design a new periodic copper structure for CO reduction with a theoretical faradaic efficiency of 97%.

We use ab initio calculations to explore the geometry, bonding and electronic properties of Mxene/blue phosphorene (BLP) heterobilayers. Perfect lattice-matched and energetically stable Mxene/BLP heterobilayers are firstly predicted to be vertically stacked with less than 1% lattice mismatch. The electronic properties of the heterobilayers are consistent with the substrate Mxene and the states projected on the isolated components are preserved. The unchanged electronic properties for the components upon the formation of the heterobilayers indicate the physical vdW interaction between the BLP monolayers and Mxene sheet, instead of the chemical bonds. The most stable BLP/Y2CX2 (X = O, and Y = Hf, Zr) are found to be a semiconductor with a type-II band alignment where the excited electrons and holes are localized in different layers. However, for BLP/Y2CX2 (X = F, and Y = Hf, Zr), the most stable structure is metallic with a strong band bending which lead to the existence of a partial flat band along the Γ point to the M point that mainly originates from the BLP monolayes. The appearance of the spatial separation of electron and hole and the partial flat band in these heterostructures have potential applications in optoelectronic devices and strong electron-electron correlation field. Our analysis also suggests that Mxene is a promising substrate to grow BLP monolayer epitaxially.

Author(s): Liu, YY; Liu, F; Wang, R; Luo, JW; Jiang, X; Huang, R; Li, SS; Wang, LW | Abstract: The complexity of charge trapping in semiconductor devices, such as high-κ MOSFETs, is increasing as the devices themselves become more complicated. To facilitate research into such charge-trapping issues, here we propose an optimized simulation framework that is composed of density-functional theory (DFT) for electronic structure calculation and Marcus theory for the calculation of charge-trapping rates. The DFT simulations are either carried out or corrected by using the Heyd-Scuseria-Ernzerhof hybrid functional. Using this framework, the hole-trapping characteristics along multiple paths in Si/SiO2/HfO2 stacks are investigated, and the relative importance of each path is revealed by calculating its exact hole-trapping rate. Besides the study on crystalline stacks, we also create an amorphous stack, which is more realistic compared with experiments and real devices, to reveal more active trapping centers and to study the statistical feature of charge trapping induced by structural disorder. In addition, to seek effective measures for relieving these charge-trapping problems, the effects of hydrogen and fluorine passivations are discussed, and physical insights for improving the performance of high-κ MOSFETs are provided.

We present a fully ab initio direct quantum transport simulation of nano-interconnect containing a record number of atoms (~5000). Various defects of the nanosized copper (Cu) interconnects, including different wire-neck shapes, twin boundaries, point defects, cobalt (Co) interstitial layers, as well as the thermal effects, are investigated with an aim to reveal the largest effects in such imperfections. We also found that the classical picture of the interconnect resistance, e.g. Matthiessen's rule, might not be valid at nanoscale, due to strong quantum interferences.

Two-dimensional transition metal dichalcogenides have attracted intense interests in recent years. Existing studies have fully explored the properties of their ground-state structures, but their global energy landscapes are still not well understood. The global energy landscape is important for understanding the experimental synthesis and thermal dynamic properties as well as for discovering new phases. This work uses material genome techniques (huge global search and data mining) to explore the global energy landscapes and new phases of two-dimensional MoS2, WS2, and ReS2 monolayers at ab initio level. Our results show that their energy landscapes have two or three major funnels, each of which consists of a few local minima with hexagonal, octahedral, quadrilateral, nanoribbon structures as well as their distorted and hybrid structures. The global-minimum structures are confined in deep funnels while the higher-energy minima stay in flat funnels and can transform to other minima at high temperatures. A f...

Utilizing Pt nanoparticles of varying sizes (2–7 nm), it was found that the oxidation of 1-propanol by molecular oxygen at 60 °C to propanal at the solid–gas and solid–liquid interfaces yielded significantly different results depending on Pt particle size and alcohol surface density. The reaction rate at the solid–gas interface was found to be 1 order of magnitude greater than that at the solid–liquid interface after normalizing concentration. In addition, catalytic activity increases with the size of Pt nanoparticles for both reactions. Moreover, water substantially promoted 1-propanol oxidation in the liquid phase, yet it inhibited the reaction in the gas phase. The gas phase and liquid phase reactions are believed to undergo different mechanisms due to differing kinetic results. This correlated well with different orientations of the 1-propanol species at the solid–gas interface versus the solid–liquid interface as probed by sum–frequency generation vibrational spectroscopy (SFGVS) under reaction condi...

Theoretical study of the electronic structures of protein is a fundamental challenge in computational biochemistry due to the large size of the systems. The electronic structure of a protein is important for some of the important protein functionalities, such as photosynthesis. In this study, we explored the charge-patching method to calculate the electronic structure of polypeptides. This method generates the charge densities of the systems by patching the charge motifs calculated from small prototype systems. The method was tested on a range of polypeptides, including the glycine polypeptide in 27-ribbon, α-helix, 310-helix, and β-strand structures. After the charge density profiles of these systems were obtained, the electronic structures of these glycine polypeptides were further calculated based on density functional theory (DFT) using a folded-spectrum method. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were analyzed and compared with conventional direct DFT calculations. The charge-patching method results were found to be in good agreement with the directed DFT results.

One of the biggest challenges of Li–S batteries (LSBs) is the dissolution of polysulfides at the cathode side. An ideal LSB cathode should be able to prevent polysulfide (Li2Sn, 1 ≤ n ≤ 8) dissolution, and should have a strong binding energy, high capacity and a light weight, as well as being electrically conductive. Herein, we present our quantum chemical calculations combined with a genetic algorithm for a global structure search using a C2N nanosheet as a LSB cathode. The Gibbs free energies of isolated Li2Sn and LimSn_C2N (1 ≤ m, 1 ≤ n ≤ 8) are calculated to set forth the studies of both the voltages and stabilities of lithiation reactions in every step of the discharging process. Our results indicate that the polysulfides can adhere to the C2N host substrate in a thermodynamically stable cluster state without dissolution, and the system has an energy capacity of up to 1122.21 W h kg−1. The calculated band-structure shows that LimSn_C2N is metallic though the original C2N nanosheet has an optical gap of 1.96 eV. This shows that C2N could be a promising cathode material for LSBs.

A new type of two-dimensional (2D) material, phosphorene oxides (POs) in four surface oxidation states (P4O1, P4O2, P4O3 and P4O4), is evaluated as a potential cathode for sealed Li–oxygen batteries by combining the first-principles method with molecular dynamics simulations and implicit solvent models. The system can be sealed because, unlike open Li–air batteries, there is no need to take oxygen from air. Our simulations reveal that: (1) by forming a maximum number of Li–O bonds, Li can be tightly chemisorbed on the surface of POs with a large binding energy ranging from −2.32 to −3.72 eV and a charge transfer of ∼0.9e from Li to O. (2) The diffusion of Li on POs is strongly anisotropic as the barrier along the armchair direction is nearly two times higher than that along the zigzag direction. The barrier dramatically decreases to 0.2 eV with increasing oxidation, indicating a high diffusivity. (3) For the P4O4 cathode, the thermodynamically stable, fully discharged product has a stoichiometry of Li : P : O = 1 : 1 : 1, achieving a specific capacity of 570.56 mA h g−1, an average open-circuit voltage of 2.55 V and an energy density of 1457 W h kg−1. The energy density can be further increased by considering solvent effects. More polar cyclic ethylene carbonate (EC) solvent brings about a larger energy density enhancement than less polar linear 1,2-dimethoxyethane (DME). (4) Semiconductor-to-metal transition occurs within POs after Li adsorption, facilitating the electrical transport in PO-based batteries. A combination of strong Li adsorption, high diffusivity, and good conductivity makes POs great candidates for future fully sealed Li–oxygen batteries.

β-Ga2O3 has drawn recent attention as a state-of-the-art electronic material due to its stability, optical transparency and appealing performance in power devices. However, it has also found a wider range of opto-electronic applications including photocatalysis, especially in its porous form. For such applications, a lower band gap must be obtained and an electron-hole spatial separation would be beneficial. Like many other metal oxides (e.g. Al2O3), Ga2O3 can also form various types of porous structure. In the present study, we investigate how its optical and electronic properties can be changed in a particular porous structure with stoichiometrically balanced and extended vacancy channels. We apply a set of first principles computational methods to investigate the formation and the structural, dynamic, and opto-electronic properties. We find that such an extended vacancy channel is mechanically stable and has relatively low formation energy. We also find that this results in a spatial separation of the electron and hole, forming a long-lived charge transfer state that has desirable characteristics for a photocatalyst. In addition, the electronic band gap reduces to the vis-region unlike the transparency in the pure β-Ga2O3 crystal. Thus, our systematic study is promising for the application of such a porous structure of β-Ga2O3 as a versatile electronic material.

Water splitting to generate hydrogen and oxygen gas is critical to renewable energy technologies, including fuel cells and rechargeable metal–air batteries. The oxygen evolution reaction (OER) has long been the bottleneck of water splitting because of its high overpotential (η) and sluggish kinetics, and development of efficient, stable, and non-noble-metal-based OER catalysts has been an extensively studied topic. Here, we propose short hydrogen bonds on reconstructed nanocrystal surface to enhance oxygen evolution activity by investigating three types of phase structures (βII, βI, and γ0) of Li2CoSiO4 (LCS) nanoparticles as OER electrocatalysts. Among them, the βII-LCS outperforms the previously reported Co-based catalysts and the state-of-the-art IrO2 catalyst for OER in the alkaline condition. Our experiments combined with ab initio calculations indicated that due to the line-linked arrangement of Co active sites at the surface of βII-LCS, short hydrogen bonds (2.54 A) are formed and linked into a net...

Oxygen evolution reaction (OER) is an obstacle to the electrocatalytic water splitting due to its unique four-proton-and-electron-transfer reaction process. Many methods, such as engineering heterostructure and introducing oxygen vacancy, have been used to improve the catalytic performance of electrocatalysts for OER. Herein, the above two kinds of regulation are simultaneously realized in a catalyst by using unique ion irradiation technology. A nanosheet structured NiO/NiFe2 O4 heterostructure with rich oxygen vacancies converted from nickel-iron layered double hydroxides by Ar+ ions irradiation shows significant enhancement in both OER and hydrogen evolution reaction performance. Density functional theory (DFT) calculations reveal that the construction of NiO/NiFe2 O4 can optimize the free energy of O* to OOH* process during OER reaction. The oxygen vacancy-rich NiO/NiFe2 O4 nanosheets have an overpotential of 279 mV at 10 mA cm-2 and a low Tafel slope of 42 mV dec-1 . Moreover, this NiO/NiFe2 O4 electrode shows an excellent long-term stability at 100 mA cm-2 for 450 h. The synergetic effects between NiO and NiFe2 O4 make NiO/NiFe2 O4 heterostructure have high conductivity and fast charge transfer, abundant active sites, and high catalytic reactivity, contributing to its excellent performance.

As the high throughout calculations and material genome approaches become more and more popular in material science, the search for optimal ways to predict atomic global minimum structure is a high research priority. This paper presents a fast method for global search of atomic structures at ab initio level. The structures global optimization (SGO) engine consists of a high-efficiency differential evolution algorithm, accelerated local relaxation methods and a plane-wave density functional theory code running on GPU machines. The purpose is to show what can be achieved by combining the superior algorithms at the different levels of the searching scheme. SGO can search the global-minimum configurations of crystals, two-dimensional materials and quantum clusters without prior symmetry restriction in a relatively short time (half or several hours for systems with less than 25 atoms), thus making such a task a routine calculation. Comparisons with other existing methods such as minima hopping and genetic algorithm are provided. One motivation of our study is to investigate the properties of magnetic systems in different phases. The SGO engine is capable of surveying the local minima surrounding the global minimum, which provides the information for the overall energy landscape of a given system. Using this capability we have found several new configurations for testing systems, explored their energy landscape, and demonstrated that the magnetic moment of metal clusters fluctuates strongly in different local minima.

CH3NH3PbI3 is one of the most promising candidates for cheap and high-efficiency solar cells. One of its unique features is the long carrier diffusion length (>100 μm), but its carrier mobility is rather modest. The nature of the mobility is unclear. Here, using nonadiabatic wave function dynamics simulations, we show that the random rotations of the CH3NH3 cations play an important role in the carrier mobility. Our previous work showed that the electron and hole wave functions were localized and spatially separated due to the random orientations of the CH3NH3 cations in the tetragonal phase. We find that the localized carriers are able to conduct random walks due to the electrostatic potential fluctuation caused by the CH3NH3 random rotations. The calculated electron mobilities are in the experimentally measured range. We thus conclude that the carrier mobility of CH3NH3PbI3 is likely driven by the dynamic disorder that causes the fluctuation of the electrostatic potential.

Pt nanoclusters play an important role in catalysis-related applications. Essential to their activities are their geometries and energy landscapes. In this work, we studied the energy landscapes of Pt clusters using a parallel differential evolution optimization algorithm and an accelerated ab initio atomic relaxation method, which allowed us to explore unprecedentedly large numbers of geometry local minima at ab initio level. We found many lower-energy isomers with low symmetry in their geometry. The energy landscapes were demonstrated to be glass-like with a large number of local minimum structures close to the global minimum. The electronic and magnetic properties of most glass-like local minima were dramatically different from the global minimum, and they should be observed in the experimental measurements. The connections between these local minima were further analyzed using data mining techniques.

LS3DF, namely linear scaling three-dimensional fragment method, is an efficient linear scaling ab initio total energy electronic structure calculation code based on a divide-and-conquer strategy. In this paper, we present our GPU implementation of the LS3DF code. Our test results show that the GPU code can calculate systems with about ten thousand atoms fully self-consistently in the order of 10 min using thousands of computing nodes. This makes the electronic structure calculations of 10,000-atom nanosystems routine work. This speed is 4.5–6 times faster than the CPU calculations using the same number of nodes on the Titan machine in the Oak Ridge leadership computing facility (OLCF). Such speedup is achieved by (a) carefully re-designing of the computationally heavy kernels; (b) redesign of the communication pattern for heterogeneous supercomputers .

The formation energies and charge-transition levels of intrinsic point defects in lead halide perovskite CsPbBr3 are studied from first-principles calculations. It is shown that the formation energy of dominant defect under Br-rich growth condition is much lower than that under moderate or Br-poor conditions. Thus avoiding the Br-rich condition can help to reduce the defect concentration. Interestingly, CsPbBr3 is found to be highly defect-tolerant in terms of its electronic structure. Most of the intrinsic defects induce shallow transition levels. Only a few defects with high formation energies can create deep transition levels. Therefore, CsPbBr3 can maintain its good electronic quality despite the presence of defects. Such defect tolerance feature can be attributed to the lacking of bonding-antibonding interaction between the conduction bands and valence bands.

The crystalline-Si/amorphous-SiO2 (c-Si/a-SiO2) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO2 interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containing Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO2, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO2 interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO2 and c-Si/a-SiO2 interfaces, the band offset significantly deviates from the experimental values by about 1 eV.

Liquid lithium is often used as a coating material in fusion reaction chambers, where it is under constant bombardment from high speed neutrons and protons. However, numerous fundamental questions are unanswered, for example whether a single proton impact can cause Li atom sputtering, and what is the electron excitation energy profile after a collision particularly for extremely high energy projectiles. Herein, we use a real-time dependent density functional method to study these questions for proton energies in the range of 30 eV to 1 MeV. The calculated stopping power agrees well with experiment, and it is found that the stopping power cannot be described by the single electron exciting spectrum based on the adiabatic eigen energies, and Li atom sputtering is not observed within our simulation time.

Critical to the development of all-solid-state lithium-ion batteries technology are novel solid-state electrolytes with high ionic conductivity and robust stability under inorganic solid-electrolyte operating conditions. Herein, by using density functional theory and molecular dynamics, a mixed oxygen-sulfur-based Li-superionic conductor is screened out from the local chemical structure of β-Li

Low-energy impurity diffusion in a host material is often regarded as an adiabatic process, characterized by its adiabatic potential energy barrier. Here we show that the diffusion process in semiconductors can involve nonadiabatic electron excitations, rending it to be a more complicated process. Impurity diffusion in a device at working temperature can pump one electron up from localized impurity state into the host conduction band and causes the impurity to be a dynamic donor since it temporarily loses its electron to the host. This nonadiabatic process, against a common belief, fundamentally change the diffusion behavior, including its barrier height and diffusion path. Although we mainly demonstrate this process with Au metal impurity in bulk Si through time-dependent density functional theory simulations, we believe this could be a rather common phenomenon as it is shown that the similar phenomena also exist in Zn, Cd impurities diffusion in bulk Si, and Ti diffusion in $\mathrm{Ti}{\mathrm{O}}_{2}$. We believe this study can open up a new direction of inquiry for such diffusion behavior in semiconductor.

Plasmon photochemistry can potentially play a significant role in photocatalysis. To realize this potential, it is critical to enhance the plasmon excited hot carrier transfer and collection. However, the lack of atomistic understanding of the carrier transfer across the interface, especially when the carrier is still "hot", makes it challenging to design a more efficient system. In this work, we apply the nonadiabatic molecular dynamics simulation to study hot carrier dynamics in the system of a Au nanocluster on top of a GaN surface. By setting up the initial excited hole in Au, the carrier transfer from Au to GaN is found to be on a subpicosecond time scale. The hot hole first cools to the band edge of Au d-states while it transfers to GaN. After the hole has cooled down to the band edge of GaN, we find that some of the charges can return back to Au. By applying different external potentials to mimic the Schottky barrier band bending, the returning charge can be reduced, demonstrating the importance of the internal electric field. Finally, with the understanding of the carrier transfer's pathway, we suggest that a ZnO layer between GaN and Au can effectively block the "cold" carrier from returning back to Au but still allow the hot carrier to transfer from Au to GaN.

The search for high efficiency and low-cost catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is vital to overall water splitting. In this work, on the basis of first-principles calculations, we screened a series of late transition metal atoms supported on a C9N4 monolayer (TM@C9N4, where TM represents Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt) as electrocatalysts for both the HER and OER. Our results demonstrate that the TM atoms can be bonded with the nitrogen atoms around the hole to form stable structures, and the bonded TM atoms are stable against diffusion. Co@C9N4 exhibits high catalytic activity toward the HER. In particular, the N active sites in the Co@C9N4, Ni@C9N4, and Pt@C9N4 systems demonstrate relatively high performance for the HER. However, Co@C9N4 and Pt@C9N4 exhibit low OER activities with large overpotentials. Among the ten cases of TM@C9N4 considered here, only Ni@C9N4 performs as a promising bifunctional electrocatalyst with N and Ni atoms as catalytic active sites for the HER and OER, with a calculated hydrogen adsorption Gibbs free energy (ΔGH*) of -0.04 eV and an OER overpotential (ηOER) of 0.31 V. The results demonstrate that TM@C9N4 is a promising single-atom catalytic system, which can be used as the non-noble metal bifunctional electrocatalyst for overall water splitting.

The condensation of half-light half-matter exciton polaritons in semiconductor optical cavities is a striking example of macroscopic quantum coherence in a solid state platform. Quantum coherence is possible only when there are strong interactions between the exciton polaritons provided by their excitonic constituents. Rydberg excitons with high principle value exhibit strong dipole-dipole interactions in cold atoms. However, polaritons with the excitonic constituent that is an excited state, namely Rydberg exciton polaritons (REPs), have not yet been experimentally observed. Here, for the first time, we observe the formation of REPs in a single crystal CsPbBr3 perovskite cavity without any external fields. These polaritons exhibit strong nonlinear behavior that leads to a coherent polariton condensate with a prominent blue shift. Furthermore, the REPs in CsPbBr3 are highly anisotropic and have a large extinction ratio, arising from the perovskite's orthorhombic crystal structure. Our observation not only sheds light on the importance of many-body physics in coherent polariton systems involving higher-order excited states, but also paves the way for exploring these coherent interactions for solid state quantum optical information processing.

While a large variety of organic and molecular materials have been found to exhibit charge memory effects, the underlying mechanism is not well-understood, which hinders rational device design. Here, we study the charge retention mechanism of a nanoscale memory system, an organic monolayer on a silicon substrate, with Au nanoparticles on top serving as the electrical contact. Combining scanning probe imaging/manipulation and density functional simulations, we observe stable charge retention effects in the system and attributed it to polaron effects at the amine functional groups. Our findings can pave the way for applications in nonvolatile memory, neuromorphic computing, and high dielectric breakdown devices. U.S. Department of Energy DE-AC02-05CH11231 U.S. National Science Foundation CHE-1300180 Marie Curie FP7 ILSES 612620 Nanotwinning FP7 NN294952

We investigate laser-induced ultrafast demagnetization of ferromagnetic systems for high-speed data processing and storage., Despite more than 20 years of development, the underlying physics of the laser-induced demagnetization process is still debated. We present a fast, real-time time-dependent density functional theory (rt-TDDFT) algorithm together with the phenomenological atomic Landau-Lifshitz-Gilbert model to investigate this problem. Our Hamiltonian considers noncollinear magnetic moment, spin-orbit coupling (SOC), electron-electron, electron-phonon, and electron-light interactions. The algorithm for time evolution achieves hundreds of times of speedup enabling calculation of large systems. Our simulations yield a demagnetization rate similar to experiments. We found that (i) the angular momentum flow from light to the system is not essential and the spin Zeeman effect is negligible. (ii) The phonon can play a role but is not essential. (iii) The initial spin disorder and the self-consistent update of the electron-electron interaction play dominant roles and enhance the demagnetization to the experimentally observed rate. The spin disorder connects the electronic structure theory with the phenomenological three-temperature model.

Author(s): Kang, J; Wang, LW | Abstract: The mixed quantum-classical nonadiabatic molecular dynamics (NAMD) is a powerful tool to study many phenomena, especially ultrafast carrier transport and cooling. Carrier decoherence and detailed balance are two major issues in NAMD. So far, there is no computationally inexpensive approach to incorporate both effects. While the decoherence effect can be easily included in the state density matrix formalism and the detailed balance can be included in surface hopping or the wave function collapse approach, it is difficult to include both of them in a unified formalism. In this work we introduce a state density matrix formalism (referred to as P-matrix) including both the decoherence and detailed balance effects for NAMD. This method is able to explicitly treat the decoherence between different pairs of adiabatic states. Moreover, the off-diagonal density matrix elements are divided into two parts, corresponding to energy-increasing and energy-decreasing transitions. The detailed balance is then enforced by a Boltzmann factor applied to the energy-increasing transition part. The P-matrix formalism is applied to study hot-hole cooling and transfer processes in Si quantum dot (QD) systems. The calculated hot-carrier relaxation time is consistent with experiments. In a QD-pair system, the hot-hole cooling time shows weak dependence on the QD spacing. However, the hot-carrier transfer rate from one QD to another is found to decrease exponentially with the QD-QD distance. When the QD spacing is small (∼1 nm), the hot-carrier transfer can be very efficient. It is also shown that the explicit treatment of decoherence time is important in order to treat this hot-carrier transfer correctly.

Author(s): Liu, YY; Zheng, F; Jiang, X; Luo, JW; Li, SS; Wang, LW | Abstract: Accurate microscopic description of the charge-trapping process from semiconductor to defects in the dielectric-oxide layer is of paramount importance for understanding many microelectronic devices such as complementary metal-oxide-semiconductor (CMOS) transistors, as well as electrochemical reactions. Unfortunately, most current microscopic descriptions of such processes are based on empirical models with parameters fitted to experimental device performance results or simplified approximations like the Wentzel-Kramers-Brillouin (WKB) method. Some critical questions are still unanswered, including: What controls the charge-hopping rate, the coupling strength between the defect level to semiconductor level, or the energy difference? How does the hopping rate decay with defect-semiconductor distance? What is the fluctuation of the defect level, especially in amorphous dielectrics? Many of these questions can be answered by ab initio calculations. However, to date, there are few ab initio studies for this problem mainly due to technical challenges from atomic-structure construction to large-system calculations. Here, using the latest advances in calculation methods and codes, we study the carrier-trapping problem using density-functional theory (DFT) based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional. The valence bond random-switching method is used to construct the crystalline-Si-amorphous-SiO2 (c-Si/a-SiO2) interfacial atomic structure, and the HSE yields a band offset that agrees well with experiments. The hopping rate is calculated with the Marcus theory, and the hopping-rate dependences on the gate potential and defect distances are revealed, as well as the range of fluctuation results from amorphous structural variation. We also analyze the result with the simple WKB model and find a major difference in the description of the coupling constant decay with the defect-semiconductor distance. Our results provide the ab initio simulation insights for this important carrier-trapping process for device operation.

Many experiments have indicated that a large polaron may be formed in hybrid perovskites, and its existence is proposed to screen the carrier–carrier and carrier–defect scattering, thus contributing to the long lifetime of the carriers. However, a detailed theoretical study of the large polaron and its effect on carrier transport at the atomic level is still lacking. In particular, how strong is the large polaron binding energy? How does its effect compare with the effect of dynamic disorder caused by the A-site molecular rotation? And how does the inorganic sublattice vibration impact the motion of the large polaron? All of these questions are largely unanswered. In this work, using CH3NH3PbI3 as an example, we implement a tight-binding model fitted from density-functional theory to describe the electron large polaron ground state and to understand the large polaron formation and transport at its strong-coupling limit. We find that the formation energy of the large polaron is around −12 meV for the case without dynamic disorder, and −55 meV by including dynamic disorder. By performing the explicit time-dependent wavefunction evolution of the polaron state, together with the rotations of CH3NH3+ and vibrations of the PbI3− sublattice, we studied the diffusion constant and mobility of the large polaron state driven by the dynamic disorder and the sublattice vibration. Two effects of the inorganic sublattice vibrations are found: on one hand, the vibration of the sublattice provides an additional driving force for carrier mobility; on the other hand, the large polaron polarization further localizes the electron, reducing its mobility. Overall, the effect of the large polaron is to slow down the electron mobility by roughly a factor of two. We believe that both dynamic disorder due to rotation of the organic molecule, and large polaron effects induced by the polarization and vibration of the inorganic sublattice, play important roles for the electronic structure and carrier dynamics of the system.

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14° and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.

Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14° and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.

In order to develop force fields (FF) for molecular dynamics simulations that retain the accuracy of ab initio density functional theory (DFT), we developed a machine learning protocol based on an energy decomposition scheme that extracts atomic energies from DFT calculations. Our DFT to FF (DFT2FF) approach provides almost hundreds of times more data for the DFT energies, which dramatically improves accuracy with less DFT calculations. In addition, we use piecewise cosine basis functions to systematically construct symmetry invariant features into the neural network model. We illustrate this DFT2FF approach for amorphous silicon where only 800 DFT configurations are sufficient to achieve an accuracy of 1 meV/atom for energy and 0.1 eV/A for forces. We then use the resulting FF model to calculate the thermal conductivity of amorphous Si based on long molecular dynamics simulations. The dramatic speedup in training in our DFT2FF protocol allows the adoption of a simulation paradigm where an accurate and problem specific FF for a given physics phenomenon is trained on-the-spot through a quick DFT precalculation and FF training.

Liquid-phase TEM study reveals the reversible giant deformation of a semiconductor nanocrystal during superlattice transitions., The behavior of individual nanocrystals during superlattice phase transitions can profoundly affect the structural perfection and electronic properties of the resulting superlattices. However, details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation. Here, we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement. Real-time high-resolution imaging with liquid-phase transmission electron microscopy reveals that following ligand removal, the individual PbSe nanocrystals experience drastic directional shape deformation when the spacing between nanocrystals reaches 2 to 4 nm. The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach. The large deformation, which is responsible for the structural defects in the epitaxially fused nanocrystal superlattice, may arise from internanocrystal dipole–dipole interactions.

We introduce a general surface passivation mechanism for cesium lead halide perovskite materials (CsPbX

As the characteristic lengths of advanced electronic devices are approaching the atomic scale, ab initio simulation method, with full consideration of quantum mechanical effects, becomes essential to study the quantum transport phenomenon in them. The widely used non-equilibrium Green’s function (NEGF) combined with the density functional theory (DFT) approach prefers a localized basis set. As many states of the art DFT calculations for solid state systems are carried out in plane waves, it is thus worth to investigate the feasibility of using the plane wave basis set for large-scale quantum transport calculations. Here we present a plane wave method for large-scale transport calculations based on a previously developed scattering state calculation approach (Wang, 2005). We address the unique computational challenges of applying that approach for large-scale systems where it is too expensive to calculate all the occupied eigenstates of the system as in conventional DFT calculations. By applying several high-efficiency parallel algorithms, including linear-scaling DFT algorithm, folded spectrum method, and Chebyshev filter technique, we demonstrate that it is possible to use this approach to simulate a system with several thousand atoms on high performance computing platforms. This method is not only used to study nanowire interconnects, showing how the shape and point defect affects their transport properties, but also used to study nanoscale Si transistor. Such quantum transport simulation method will be useful for investigating and designing nanoscale devices.

New structures and compositions as inorganic solid-state electrolytes (ISSEs) are needed for all-solid-state lithium batteries (ASSLBs). Here we report the theoretical discovery of three new structure types by computational identification of nearest neighbor Li–Li distance (nRLi-Li) within Li-ion sublattice of fluorides which correlates with ISSEs ionic conductivity. This is achieved by computing Li-ions radial distribution functions and whose first peak corresponds to nRLi-Li. Subsequent theoretical exploration of the four fluorides with small nRLi-Li affords three materials (Li3MF6 (M = Al, Sc, Ga)) with crystal structures featuring three-dimensional diffusion channels for Li-ions. The three fluoride materials exhibit very well (electro)chemical stability and high ionic conductivity. The screening method used in this work will accelerate the systematic discovery of high ionic conductivity ISSEs for use in ASSLBs and related applications.

Cu-doping into Zn 1−x Cd x S can greatly enhance the photocatalytic H 2 evolution from water splitting under visible-light irradiation. However, it is still controversial for how the Cu-dopant improves this performance. Here, we report that appropriate Cu-doped Zn 0.5 Cd 0.5 S nanocrystals reach 21.4 mmol/h/g of H 2 evolution rate without cocatalyst in the visible-light region, which is also 2.8 times as high as that of the undoped counterpart, and the corresponding apparent quantum efficiency is 18.8% at 428 nm. It is firstly confirmed that the Cu 2+ changes into Cu + after being doped by soft X-ray absorption spectroscopy (sXAS). We theoretically propose that the transformation of 2Cu 2+ to 2Cu + results in one adjacent S 2 − vacancy (V S ) in host during the doping process, while the Cu + -dopant and V S attract the photoexcited holes and electrons, respectively. Accordingly, the photocatalytic activity is improved due to the enhanced separation of photoexcited carriers accompanied with the enhanced light absorption resulting from the Cu + -dopant and 2Cu + /V S complex as possible active site for photocatalytic H 2 evolution.